In recent years, in an imaging device using a solid-state image sensor such as a CCD or a CMOS image sensor, a filter that shields light in a near-infrared wavelength range, a near infrared cut filter, has been used for the purpose of well reproducing a color tone and obtaining a clear image.
On the other hand, unlike the near-infrared cut filter, a filter that shields light in a visible wavelength range (to be also referred to as visible light hereinafter) and transmits light in an infrared wavelength range (to be also referred to as infrared light hereinafter) to increase sensitivity of the infrared light, an infrared transmission filter, has also been used mainly for an infrared camera, an infrared sensor, and the like, and demands for the infrared transmission filter excellent in infrared transmitting property/visible light shielding property, and an infrared camera, an infrared sensor, and the like using this have also been increasing.
As the infrared transmission filter, for example, in the patent document WO 2014/084147 A1 discloses a filter using four pigments (red/yellow/blue/purple pigments) and having an average transmittance of light in the wavelength range of 825 to 1300 nm being 99% or more. On the other hand, its average transmittance of light in the wavelength range of 400 to 700 nm is about 6% and thus is not low at all.
The patent document JP-A 2014-130332 discloses an infrared transmission typed optical filter using a perylene-based compound and two types of colorants (a blue or green colorant and a yellow or red colorant) in combination, and an optical sensor using it.
However, even this optical filter has a transmittance of light in the wavelength range of 700 nm being about 4 to 7%, and thus the shielding property against visible light is insufficient, resulting in that it is impossible to obtain a highly sensitive optical sensor.
As above, in the conventional infrared transmission filter, particularly, the shielding property of visible light is not sufficient, and further in a spectral transmittance curve, a gradient (rising) of change in transmittance from the visible wavelength range to the infrared wavelength range is gentle. Therefore, the light in an unnecessary wavelength range cannot be suppressed sufficiently, to thus be noise, so that it is not possible to achieve an infrared camera and an infrared sensor that have high sensitivity.